The Missions of Astronomy

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Harry Ransom Center, University of Texas at Austin
Aristotle, Ptolemy, and Copernicus; illustration by Stefan Della Bella from the frontispiece of Dialogo di Galileo Galilei (1632), on view in the exhibition ‘Other Worlds: Rare Astronomical Works,’ at the Harry Ransom Center of the University of Texas Austin through January 3, 2010

A few years ago, I decided that I needed to know more about the history of science, so naturally I volunteered to teach the subject. In working up my lectures, I was struck with the fact that in the ancient world, astronomy reached what from a modern perspective was a much higher level of accuracy and sophistication than any other science. One obvious reason for this is that visible astronomical phenomena are much simpler and easier to study than the things we can observe on the earth’s surface. The ancients did not know it, but the earth and moon and planets all spin at nearly constant rates, and they travel in their orbits under the influence of a single dominant force, that of gravitation.

In consequence, the changes in what is seen in the sky are simple and periodic: the moon regularly waxes and wanes, the sun and moon and stars seem to revolve once a day around the celestial pole, and the sun traces a path through the same constellations of stars every year, those of the zodiac. Even with crude instruments these periodic changes could be and were studied with a fair degree of mathematical precision, much greater than was possible for things on earth like the flight of a bird or the flow of water in a river.

But there was another reason why astronomy was so prominent in ancient and medieval science. It was useful in a way that the physics and biology of the time were not. Even before history began, people must have used the apparent motion of the sun as at least a crude clock, calendar, and compass. These functions became much more precise with the introduction of what may have been the first scientific instrument, the gnomon, attributed by the Greeks variously to Anaximander or to the Babylonians.

The gnomon is simply a straight pole, set vertically in a flat, level patch of ground open to the sun’s rays. When during each day the gnomon’s shadow is shortest, that is noon. At noon, the gnomon’s shadow anywhere in the latitude of Greece or Mesopotamia points due north, so all the points of the compass can be permanently and accurately marked out on the ground around the gnomon. Watching the shadow from day to day, one can note the days when the noon shadow is shortest or longest. That is the summer or the winter solstice. From the length of the noon shadow at the summer solstice one can calculate the latitude. The shadow at sunset points somewhat south of east in the spring and summer, and somewhat north of east in the fall and winter …

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